Credit: The Associated Press

FILE - In this March 3, 2017 file photo, U.S. Rep. Niki Tsongas, D-Mass., participates in a small-business roundtable discussion in Lawrence, Mass. Tsongas is retiring after five terms in Congress, so ten Democrats and one Republican are vying for their parties' nomination for the Third District seat in the Tuesday, Sept. 4, 2018 primary. (AP Photo/Elise Amendola, File)

An open Massachusetts congressional seat has prompted a political free-for-all of sorts, with 10 Democrats and a Republican vying for the affection of voters in Tuesday's primary — putting it among the nation's most crowded House races.

Democratic Rep. Niki Tsongas, the widow of former U.S. Sen. Paul Tsongas, is retiring after five terms and hasn't endorsed any potential successor.